The general objectives of this research project concern the delineation of the precise mechanisms of cooperative interactions between thymus-derived (T) lymphocytes and the bone marrow-derived (B) cell precursors of antibody-forming cells in the development of immune responses. These studies are focused primarily on the responses to hapten-carrier conjugates both in vivo and in vitro, and encompass analyses of antibody responses of various immunoglobulin classes including IgM, IgG and IgE, from the standpoint of the cells involved in the development of such responses, genetic control of their functions, and the role of certain biologically-active lymphocyte-derived mediators in cell triggering and differentiation. These studies are ultimately directed toward the adaptation to potential therapeutic applications, the capacity to greatly enhance or suppress humoral or cell-mediated immune responses to a variety of antigens.